A high number of methods--from cooling the items (products) to the use of plastic foils, wrappings, combined package materials with various adsorption capacity for specific gas permeability became--known in order to retard the ripening and for preserving the freshness, taste and texture of rapidly perishable fresh fruits and other vegetables. In many cases, the fruits or vegetables to be stored are treated by germicidal agents, microwave irradiation or chemicals.
Due to the hygienic prescriptions continuously becoming more and more severe it is aimed to retard the ripening of products by combining adsorptive substances of natural origin (e.g. activated carbon) with traditional package materials such as cardboard boxes and simultaneously to omit the treatment of the product to be stored in the packing of rapidly perishable foods, particularly fruits and vegetables.
It is also known that during the ripening and storage of fruits various amounts of gases and vapours having various compositions are liberated, by the enrichment of which in a closed container the ripening is accelerated and therefore, a rapid deterioration of the products to be stored is induced. Adsorbents applied to the space closed by the storage unit or container e.g. zeolites, activated carbon, silica gel, resp., are capable of retarding the ripening, or deterioration of items to a certain extent by the partial binding of the gases and vapours. However, the adsorbents used cannot be applied durably onto the surface of the known fruit package materials therefore, their use is cumbersome and laboursome.
It has also become known that the mixed potassium, sodium, magnesium or ammonium salts of humic acid as well as of nitrohumic acid prepared by the reaction of humic acid with nitric acid, which are utilized in the therapy and in the cosmetic industry, possess some activity in inhibiting rot and zymosis, and thus, they can be utilized as adsorbents applied with the package materials in the storage of fruits. The salts of humic and particularly nitrohumic acid have been used in a form filled onto or applied to felt, polyethylene foam net or paper bag and a certain grade of freshness-preserving effectivity was observed in the case of storage of fruits [C. A. 111, 76736 (1989); Japanese Pat. 64. 02,528].